The use of ultrasonic aspiration equipment for surgical procedures is well known in the art. U.S. Pat. No. 3,589,363 disclose ultrasonic aspiration for use in removing cataracts. U.S. Pat. No. 4,223,676 relates to its use for the removal of neoplastic tissue and U.S. Pat. No. 4,750,902 includes endoscopic procedures for bladder tumor and stone removal.
These devices have proven to provide great utility in medical surgical practice. Soft tissue is athermally dissected, leaving parent tissue undamaged. Necrosis, common to cryosurgical, electro-surgical and laser procedures is minimized in ultrasonic surgery because cell destruction is confined to a single layer. Elastic, connective tissue, however, is resistant to ultrasonic attack. For example, blood vessels having diameters larger than 1 millimeter are normally not severed by ultrasonic aspirators. In prostatectomies, the benign gland can be entirely removed without effect to the prostatic capsule (Krawitt et al., Ultrasonic Aspiration of Prostate, Bladder Tumors and Stones, Urology, 30:6 (1987) pp. 578-580). Tumors of the spinal cord can also be dissected and aspirated while preserving the anatomical and physiological integrity of adjacent neural tissue. Histologic assays of ultrasonically aspirated tissue have shown preservation of cellular morphology, enabling pathological analysis of specimens to be made with confidence (Richmond et al., Evaluation of the Histopathology of Brain Tumor Tissue Obtained by Ultrasonic Aspiration, Neurosurgery, 13:4 (1983), pp. 415-419).
As a result of the advantages attendant to the ultrasonic technique, subjects receiving such procedures have reported more rapid recovery and better retention of normal function than populations receiving conventional treatment (Malloy et al., Endoscopic Ultrasonic Aspiration of the Prostate, Bladder Tumors and Stones, Journal of Urology Supplement, May, 1989). In some cases, such as the surgical management of astrocytomas, ultrasonic aspiration is the only known method for removal that is both safe and effective (Epstein et al., Surgical Management of Extensive Intramedullary Spinal Cord Astrocytoma in Children, Concepts in Pediatric Neurosurgery, 2, (1982) pp. 29-44). The application of this technology, initially in ophthalmic and neurosurgery, has consequently grown to embrace urologic, general and cardiovascular surgery, where, recently, the successful debridement of calcified heart valves has been demonstrated (Sternlieb et al., Ultrasonic Restoration of Severely Calcified Aortic Valve, The Lancet, Aug. 20, 1988, p. 446).
U.S. Pat. No. 4,750,902 includes a comprehensive review of the art and literature forming the foundation of the technology. In essentially all indicated applications, the instrumentation excites and sustains controlled extensional resonance of slender, hollow, prismatic tubes, thereby producing a standing wave whose principal attribute of interest is reciprocal motion of a surgical tip. The frequency of vibration, determined by the dimensions of the tube and the electro-mechanical transducer exciting the motion, is typically selected to lie within the range of 10 to 50 kHz (10,000 to 50,000 cycles of vibration per second). It has been discovered that, if the magnitude of the vibration is adequately large within this frequency band, the application of the tip directly to soft tissue, such as muscle, produces separation of the cellular structure at the locus of contact. The peak to peak vibration amplitude required to produce the phenomena depends upon the particular tissue under consideration, but usually lies within the range of 6 to 18 mils (0.006 to 0.018 inches or 150 to 460.times.10.sup.-6 m). If a source of vacuum is simultaneously applied to the bore of the hollow tip, tissue parted by the vibration can be separated and withdrawn into a suitable collection vessel.
The agent responsible for the observed phenomena is cavitation of intercellular water, or the free water between cells. Cavitation is well known for causing the erosion of apparatus such as ship propellers for example. Cavitation may also be used to advantage in ultrasonic cleaning apparatus. In surgical applications, the free intercellular water enters a vapor phase, manifest as micron (10.sup.-6 m) sized bubbles, as the tip rapidly retracts during one half cycle of vibration. When the tip returns in the next half cycle, the bubbles collapse, producing extraordinarily high but very localized pressure. Typically, the pressures produced are on the order of one million atmospheres. Cell walls adjacent to the tip are ruptured in the process, producing the observed dissection.
Because the effect is greatly dependent upon free water content, ultrasonic aspiration can differentiate between tissues of different hydration. For example, tumors can be dissected directly off the carotid artery and parenchyma can be separated from the vascular web in the renal pelvis. For the same reasons, the prostate can be entirely enucleated leaving both the prostatic capsule and bladder neck intact. Connective tissue such as that composing blood vessel walls, encapsulating membranes and fascia have much lower intercellular water content than the tissues they bind or contain or to which they are otherwise attached. The distinctive ability of ultrasonic vibration to discriminate and the confinement of cell destruction to one layer have thus secured acceptance of the technology in important surgical procedures.
Although dissection and aspiration using a blunt, hollow and intensely vibrating tube have demonstrated significant surgical utility, their use is limited precisely by the very effect they exploit: tissues having little hydration are extremely resistant to attack. For example, in surgery of the knee, where the meniscus or synovium must be partially removed to restore function following an injury, this technology currently offers no competition to the scalpels or other cutting devices available to perform the procedure. The same situation prevails regarding the discs of the spinal cord. In general, within the body, those structures intended to absorb physical abuse from exertion are difficult to excise surgically. It is in these specialties of surgical practice that ultrasonic aspiration has been notably unsuccessful
Another limitation of current ultrasonic instruments involves their restricted ability to cleanly dissect the "cores" of tissue that are produced from the parent structure. This difficulty is particularly noticeable when the subject anatomy is perpendicular or at an acute angle to the tip. Tissue filling the tip bore can not easily be separated without angling the tip to sever the "pedestal" attachment, and, in certain procedures, anatomical restrictions do not permit such movement. An example is the aspiration of the pituitary gland, which is located at the base of the brain. The inability of the straight ultrasonic tip to completely remove this portion of the gland through an opening made in the roof of the mouth is, in part, related to its acute presentation.
Another limitation of current ultrasonic instruments is apparent in endoscopic procedures, where the surgeon's view is provided by a telescope and where perspective is extremely important. The surgeon must be able to gauge the position of the cutting implement in relation to the entire target. The spatially fixed relation between the ultrasonic tip and telescope lens such as that disclosed in U.S. Pat. No. 4,750,902 does not provide such a perspective. A portion of the field of view is always blocked by the tip, which must, of necessity, remain visible, and it is not possible to extend the tip into the field to judge its size in relation to associated anatomy. The surgeon is thus forced to operate "right in front of his nose."
U.S. Pat. No. 3,526,219 illustrates the evident ability of ultrasonic vibration to enhance cutting by applying vibration to a number of knife tips attached to an ultrasonic transducer. In this use of vibration, cavitation plays no rule whatsoever in dissection. It is rather the addition of reciprocal motion to the blade edge that enhances penetration into tissue. However, all ultrasonic aspirators utilize a tube whose opening is at nearly a right angle to its axis and to the direction of application. If the opening is bevelled, penetration into tissue is facilitated but core pedestals remain more difficult to sever.
Also of interest to the surgeon is the possibility of providing an electrocauterizing radio frequency potential to the ultrasonic tip. The currents produced by such potentials, when passed from the tip through tissue to a return electrode, have long been known to effectively seal bleeding vessels. U.S. Pat. No. 4,750,902 discloses one way for providing such potentials to the tip of ultrasonic aspirators. Others have evaluated the use of electrical coagulating currents in the endoscopic dissection of fibrocartilaginious structures of the knee (Caspari, Current Development of Instrumentation for Arthroscopy, Clinics in Sports Medicine, 6:3 (1987), pp. 626-627; Johnson, Arthroscopic Surgery: Principles and Practice (third edition), Verlag Springer (1986), pp. 244-245).
U.S. Pat. No. 4,838,853 discloses an ultrasonic handpiece for the removal of meniscus. The hollow tip is vibrated extensionally while a source of vacuum is connected to the tip bore to remove dissected fragments.
U.S. Pat. No. 4,504,264 discloses an ultrasonic surgical device that provides both irrigation and aspiration as well as tip rotation through a specified arc of 5 to 60 degrees. The handpiece of this patent is rather bulky and difficult to manipulate in precise surgical procedures.
Continuously rotating instruments for the removal of tissue are also shown in U.S. Pat. No. 4,203,444, where rotation of a hollow tube within a protective sheath is used with aspiration to (1) capture tissue within a window, (2) sever the entrapped specimen by rotation of the tip and (3) withdraw the dissected tissue by vacuum to a collection container.
Rotating ultrasonic transducers are also generally known in the metal working or mineral extraction fields. U.S. Pat. No. 3,614,484 shows a method for introducing continuous rotation into an extensionally vibrating ultrasonic transducer for enhanced machining of materials. The ultrasonic transducer is mounted to the rotating, non-vibrating frame at points where significant ultrasonic vibration is known to exist. The wear induced by this support limits the life of the appliance. More recently, U.S. Pat. No. 4,828,052 shows an attachment to a rotating ultrasonic transducer that permits coaxial irrigation for the improved drilling of very hard materials.
Accordingly, there is a need in the art for a surgical instrument which is capable of reducing the disadvantages of current devices.